Method of removing hardened photoresist material from printed circuit conductors



March 12, 1963 c. J. BEUscHER 3,081,203

METHOD oT REMovING HARDENRD PRoToREsIsT MATERIAL FRoM PRINTED CIRCUIT coNnucToRs Filed May ze. Iseo Piz.

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P/A/SE' IN WA TEE @//VSE /N WA 75E AND DRY ite States 081,203 METHI) OF REMVING HARDE-NEI) PHT- RESET MATERIAL FROM PRENTED CIRCUIT Ciinton i. Beuscher, Fort Wayne, Ind., assigner to inter` This invention relates generally to methods of making printed circuit boards, and more particularly to a method of removing hardened photoresist from the surface of the copper conductor of a printed circuit board to permit application of la thin coating of a metal which will displace copper, such as gold.

In accordance with a conventional method of making printed circuit boards, a relatively thin board is provided formed of epoxy resin reinforced with glass fibers and having athin layer of copper deposited on either or both sides. A photoresist emulsion purchased from the Eastman Kodak Company, Rochester, New York, and identiiied as KPR is applied on the surface of the copper layer by dipping and is then dried in an oven in darkness. This photoresist material is photosensitive and the desired circuit arrangement is printed thereon by applying a photographic negative of the circuit to the surface and exposing the board with the negative thereonto light; on the negative, the conductive elements appear as light lines whereas Ithe remaining portions 'are dark. As a result of this exposure to light, portions of the photoresist surface on which the light has impinged, i.e., the parts which ultimately are to form the conductive elements, harden. The exposed board is then placed in a developing solution which washes away that part of the photoresist material which has not hardened. The board is then placed in an etching tank in which the portions of the copper surface which are not covered by the hardened photoresist material are etched away with ferric chloride.

The copper conductors on the printed circuit board provided by the above-described method have a thin surface coating of hardened photoresist material which is lhard, scaly, and adheres tenaciously to the conductor. Due to thepresence of this hardened photorcsist material, it is difficult to make soldered connections directly to the copper conductors. It has been found desirable to apply la thin coating of gold on the surface of the copper conductors in order to 'provide better electrical contact, to inhibit corrosion of the conductors, and to provide greater solderability. However, a' gold'coating cannot be directly applied to such printed circuit boards due to the presence of the hardened photoresist coating on the conducto-rs. In the past, this hardened photoresist coating has been physicaly removed by abrading with steel wool and/ or wire brushes. However, such prior manual methods not only are time consuming and :thus expensive, but further, in the case of steel wool, small fragments tend to rip off and to become caught or snagged on the conductors, thus short circuiting the same, and in the case of wire brushes, unless great care is exercised, there is a tendency for the conductors to be physically ripped off of the board. It is therefore desirable to provide a chemical dip process applicable to quantity production for removing the hardened photoresist material from the surface of the copper conductors of a printed circuit board; quite obviously, the hardened photoresist material cannot be removed by conventionai etchants since its basic purpose is to resist acid etching in order to form the basic printed circuit board. Furthermore, no commercially available solvent has been found which will dissolve the KPR photoresist material, the constituents of which are not presently known. I have discovered, however, that the hardened photoresist material can be removed by applying thereto -a material f assigns e Patented Mar. I2, 19h31 which converts the photoresist into a gelatinous substance i subjected to a mild acid etch preparatory to application ofthe gold coating by a gold immersion dip.

It is accordingly an object of my invention to provide a method of removing hardened photoresist material from the surface of a printed circuit board conductor.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a fragmentary cross-sectional view showing a printed circuitboard with a copper conductor thereon having its surface coated with hardened photoresist material; and

FIG. 2 diagrammatically shows the steps in the method of my invention.

In accordance with my invention, I dip the printed circuit board as described above for approximately two minutes in a liquid stripping material sold by the Oakite Manufacturing Company under the identification SA, the stripping material being at room temperature. The Oakite SA stripper has a density of about 1.3 and a pH of about 1.0. The Oakite SA stripper is a solvent-acid paint remover consisting of a balanced mixture of various solvents including a chlorinated hydrocarbon with a non-ionic ether surface wetting agent, an evaporation retardant including a hydrocarbon type wax, and an organic acid including formic acid having a pil-l of 2 to 21/2. This dipping of the printed circuit board in the Oakite SA stripper converts the hardened KPR photoresist material into a gelatinous substance. As previously indicated, the constituents of the Eastman KPR photoresist material are not known, however, it is well known that egg albumen was commonly used for many years as a photoresist material -in the photoengraving art. Egg albumen is, of course, organic, with a high portein content, which would thus account for its tenacious adherence to the surface upon which it is applied. The Eastman KPR photoresist material is known to be organic, and I thus hypothesize that like egg albumen, it has a high protein content; I attribute the jelling of the hardened KPR phoztoresist material when dipped in the Oakite SA stripper to the high protein content of the Eastman KP-R photoresist material. I have found that if the board is left in the KOakite SA stripper for more than three minutes, deterioration of the epoxy-glass board results; no deterioration is observed, however, following the two minute dip.

Following dipping of the board in the @akite SA stripper, the board is washed in running water or dipped in a running water tank which removes some larger globales of the gelatinized material, it being pointed out, however, that the gelatinous material is not water soluble.

The board is then dipped in methyl alcohol (CHgOI-l, technical grade) at room temperature for approximately two minutes to four minutes, which hardens the gelatinous photoresist material. It appears that during this dipping operation, some of the methyl alcohol works underneath the gelatinous coating so that when the coating hardens, it is caused to peel away or separate from the surface of the conductor. As a result of this dipping of the board in methyl alcohol, the gelatinous coating on the conductors changes from essentially transparent to a milky coior. At the end of this methyl alcohol dip, much of the hardened gelatinous material is hanging rather loosely on the surface of the conductors and some of the hardened material iioats ofi.

3 The' board is then dipped in hexane (CGHM) which isy a thinner liquid having a specic gravity of .66, and.

which does not attack the epoxy-glass board, the hexane being at room temperature. The hexane removes substantially all of the remaining hardened gelatinous material by flotation.

It may be found desirable to then dip the board in a chelating7 agent at 165'to 185 F., such as a material manufactured by the Oakite Manufacturing Company and identified as 62, orA a ve toten percentaerosolsolution; however, I have found that the chelating agentdip is not generally necessary for satisfactory results.

With the method described above, I have found thateighty to ninety percent of the boards treatedV asdescribed above willhave substantially all of the Eastmen KPR photoresist material removed from the'surfacevof the copper conductors; In the event that someresidue of photoresist material remains. on the conductors fol-- lowing the above-.described treatment, the board can be subjected to a second treatment with the respective dip,

times cut in half.

In order to apply athin coating ofgol'd .tothe copper Inc., Waterbury, Connecticut,I and identified as L5, theboard` being subjected yto the McDermid L forapproximately two minutes. I haVealso employed for the mild etching a lmixture of approximatelyyi'itty percent( 50%) phosphoric acid (H3PO4), twenty rpercent (20%) sul'-V phuric acidl (H2504), ten` percent (10%) nitric acid (HNOa), and twenty percent (20%) water, the boardbeing dipped therein for'iifteen; to twenty seconds.

Following the mild acid etch, the' board is again washedinwater and the gold .coatingisapplied by a gold immersion dip. Here I have employed a liquid gold dis-v placement material comprising a solution ofapproximately three-quarters ounce to the gallon of forty-one percent (41%) goldpotassium cyanide (KAU(CN)2.2H2O), four ounces to the gallon kof sodium cyanid-e (NaCN) and five ounces to the gallon of sodium carbonate (Na2CO3). I have employed this aqueous solution at a temperature from 60 to 65 C., with the board being dipped therein for a period of thirty (30) seconds to one (1) minute, to thereby apply a.g oldv coating approximately .000001- .000003 inch thick. It is well understood that with such a gold vimmersion dip, that the gold is depositedl on. the conductor by disp1acement,.i.e., the gold displacing copper on the surface, with the copper goingl into .the solution.

The board Iis then given another waterV rinse and dried to complete the method of my invention.v

In production, the above chemical dip process for removing the hardened Eastman KPR photoresist material from the copper conductors has been found to be fteen to twenty times faster than the prior methods of removal of the hardened photoresist material by hard scrubbing with detergents, steel wool, and/or wire brushes.

I have found that for best results, it is necessary that the specific gravities of' the several chemical dipsemployed in practicing my invention be closely controlled. Thus, I have found thatthe specific gravityv of the Oakite SA stripper should be maintained between 1.240 and 1.320, the specilic gravity of the methyl alcohol should be maintained between .800 and .850, the specific gravity of the hexane should bemaintained between .6'60 and .700, the percentage of McDermid L5 solution should be maintained between 25% and 35% and the cyanide content of the gold immersion -dip should be maintai-ned between 1.25 and 2.25 ounces per gallon'.

While I have described above the principles of my invention in connection with specific methods, it is to be clearly understood that this description ismade only byy Way of example and notas av limitation'to thescope of my invention.

What is claimed is: l The method of removing hardened photoresist'material from the surface of a conductor of a printed'circuit board comprising the steps of:

dipping said board in a solvent-acid paint remover consisting of a balanced mixture of various solvents includingA a chlorinated hydrocarbon with a non-ionic ether surfacewetting agent, yan evaporation retardant includinga hydrocarbon type wax, and an organic acid including formic acid at room temperature for approximately two. minutes to render-said photoresist material gelatinous; d-ippingsaid board in methyl alcohol at roomy temperature for approximately two' to four minutes to harden said gelatinous'photoresist material and to separate the same from said conductor; and dipping saidv board in hexane at room temperature for approximately two minutes to remove said hardened gelatinous photoresist material by otation.

References Cited in the tile of this patent UNITED STATES PATENTS 2,338,673 Slack Ian. 4, 1944 2,417,468Y Canziani Mar. 18, 1947 2,435,239 Schub Feb. 3, 1948' 2,709,666 Speekman May 31, 1955 2,737,465 Pessel Mar. 6, 1956 2,925,332 Standley Feb. 16, 1960 

